Cooling of electronic components is becoming increasingly significant. The trend in integrated circuit (IC) design, and in particular, central processor units (CPUs), is increased speed and circuit density. This increased speed and density, in turn, causes the IC to require more power and generate more heat. Without sufficient cooling, the IC may not perform as specified and may suffer a decrease in reliability.
To cool high-powered electronic components on circuit boards, liquid or refrigerant-cooled systems may be utilized. Typically, the cooling system includes a cooling circuit, which circulates liquid or refrigerant through a heat dissipation structure that is thermally coupled to an electronic component. The circuit board may include a plurality of such high-powered electronic components, with each component thermally coupled to a separate heat dissipation structure through which liquid or refrigerant flows. The heat dissipation structure(s) may be, for example, a cold plate or an evaporator.
Special consideration must be given when coupling the cooling circuit to multiple heat dissipation structures. Electronic component heights are not regular, and can vary considerably. This height variation between electronic components typically translates directly into height variation between the multiple heat dissipation structures. Not only can device heights vary between types of devices, such as a CPU versus a high frequency switching component, but device heights can vary within a class of electronic components. For example, a particular flip-chip package may have a height tolerance of +/−0.02″. Hence, a production run of the same board must allow for both device/package tolerances, as well as interconnect height variations between different classes of electronic components.
Adding further to the complexity of coupling the cooling circuit to the heat dissipation structure(s) is that the connection between the cooling circuit and the heat dissipation structure may cause undue stress to the electronic component, the heat dissipation structure, and/or to the interface between the electronic component and the heat dissipation structure. Such stresses may damage the electronic component and/or the heat dissipation structure, or may lead to decoupling of the interface between the electronic component and the heat dissipation structure.
Conventional methods of connecting the cooling circuit to multiple heat dissipation structures include complicated piston arrangements that exert pressure onto the individual heat dissipation structures to maintain good contact pressure, or rely on expandable fluid bladders to make up for height difference in electronic components. While these methods are functional, they are not cost effective or exceptionally reliable fluid handling methodologies. To manufacture multiple pistons is expensive, and the control of individual pressures on the electronic components presents additional engineering challenges. Expandable fluid bladders are also expensive, and furthermore, are susceptible to failure, especially in the bladder mechanism, and typically are not nearly reliable as the electronics they are cooling.
Cooling circuits used in cooling electronic components, or other fluidic circuits, which may be used, for example, in heating or temperature control systems, commonly allow for temperature-driven fluid expansion and/or contraction. For optimal performance, these circuits may also have the capability to remove trapped air from the recirculating fluid, and in doing so, include extra fluid to make up for the displaced air that is removed from the recirculating fluid.
A fluid reservoir is usually provided for this purpose. Most fluid reservoirs are made from a simple tank, where returning fluid enters near the top of the reservoir, and fluid is drawn from the reservoir from the bottom of the tank. In this arrangement, trapped air floats to the surface of the tank, and the air that is removed is automatically displaced by an equivalent volume of water, drawn, via a pump, into the recirculating lines from the bottom of the tank. As the fluid heats and cools, the volume of fluid in the tank increases and decreases respectively, the volume of fluid in the recirculating lines remain constant, and the specific volume (1/density) of the fluid changes with temperature.
Traditional reservoirs work well in an environment where the direction of gravity does not change with respect to the reservoir geometry. That is, the free surface of the fluid always remains in the same orientation with respect to the tank geometry. Because of this the “bottom” and “top” of the tank remains constant with respect to the free surface of the liquid in the reservoir.
In some applications (such as liquid electronics cooling), it is advantageous that the functions of a reservoir be present in the recirculating system when operating in an environment where the direction of gravity can change with respect to reservoir geometry. However, if a traditional reservoir is inverted, the exit line from the reservoir becomes exposed to the air in the tank, filling the recirculating lines with air, and the pumping operation would cease, or at the very least, the recirculating fluid would become highly mixed with the trapped air in the entire system, seriously degrading the system's performance.